docs(readme): remove word “both”

[hongbo-miao]

What this PR does / why we need it:

Fix grammar. Initially met issue https://stackoverflow.com/questions/79446862/loki-distributed-mode-gives-error-at-least-one-bucket-name-must-be-specified and then found out loki supports all 3 modes. So no need use loki-distributed any more I guess.

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR